The present invention arose out of research sponsored by the USAF Rome Laboratory under contract number F30602-92-C-0038. The U.S. government has rights in the invention.
The present invention relates to an optical laser amplifier structure which is integrally combined with a spontaneous emission filter for substantially improving the signal to noise ratio of the amplifier.
Optical laser amplifiers are used to increase the amount of signal from a device or system in which a signal is conveyed by stimulated emission, i.e., a laser beam. These amplifiers are common in fiber communication networks and switching systems for example. The output from an optical amplifier generally consists of the amplified stimulated emission plus a certain amount of unwanted spontaneous emission. The spontaneous emission occurs as a result of random recombination of carriers in the laser amplifier: this random recombination does not contribute to the amplified stimulated emission. The spontaneous emission can be referred to as noise in the signal. Noise corresponds to the integral over wavelength of the spontaneous emission, otherwise known as the total spontaneous power. The spontaneous emission has broad bandwidth compared with the amplified signal. In the case of an optical laser amplifier having a low signal to noise ratio, a photodetector used to detect the signal will produce photocurrent that is orders of magnitude larger for the integrated spontaneous emission than for the amplified stimulated emission. In such case, it is difficult to separate the signal from the noise.
Several techniques can be used to extract the optical signal from the noise. In the laboratory, the signal can be viewed on an optical spectrum analyzer with sufficient resolution. Lock-in amplifiers can also be used. However, both of these devices are large, costly and slow. Another method uses holograms to redirect the laser light which results in spatial separation between the stimulated and spontaneous emission. However, the hologram introduces additional optical losses, and it is large and mechanically unstable. The same is true for fiber based systems. In some applications, the optical signal must be electronically reconstructed to obtain the required amplification due to the large noise accrued by cascaded optical amplifiers. This reconstruction process consists of detecting the optical signal at a point of relatively large signal to noise ratio, electronically amplifying and processing the detected signal, and then retransmitting it through a laser.
In view of the foregoing, a need therefore exists for a faster, cheaper, less complex and smaller means by which noise can be filtered from an optical laser amplifier's output signal.